Again, you need to validate your mail() command. By using @ or IF.

The mail() function returns TRUE or FALSE upon sendin the email.
In order for you to write a decent script, you will have to verify that this mail was sent corrently.

Therefore you will need to...

The following is the necessary code to store, check that no info was empty and email the user.
You will need to change the $to variable to suit your needs.
You will also need to make <input...

You could use JavaScript Encode, to encode the HTML/page source, so that it isn't readable at all.

I understand the code, it was for the C beginners trying to understand the code (as there were 0 comments in his code)

Some comments might help the beginners Aia

Just a Tip for you, as you're new to PHP.
Once you've written to the txt file and want to get all the info from it again.
You can use the file_get_contents() function to store the file's contents...

/* init */
$arr = array(
"chapter1" => array(),
"chapter2" => array(),
"chapter3" => array()
);

...

Another solution would be, instead of using a foreach() you could use a for() loop.

$arr = array("cat", "dog", "horse", "frog");

echo "<select name=dropdown_list><option value="bleh">Choose...